The Ras pathway is one of the most commonly deregulated pathways in human cancer. RAS genes are mutated in a broad spectrum of tumor types; however, RAS mutations are conspicuously absent in breast cancer, despite the fact that the pathway is hyperactivated. We have identified two RasGAP genes that appear to function as tumor suppressors in breast cancer. The goal of this application is to 1) understand how these proteins function and interact, 2) elucidate the mechanism by which these genes independently and cooperatively regulate breast cancer development, progression, and/or drug resistance, and 3) establish the frequency and clinical setting in which these genes are inactivated in human breast cancer. This work will serve as a basis for understanding why a subset of specific breast cancers progress, will elucidate novel aspects of Ras signaling, and will reveal how these new tumor suppressors link Ras to other signaling networks.